(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engine.
(2) Description of the Related Art
As measures taken against exhaust gas pollution and fuel consumption, a lean burn system has recently been developed. According to this lean burn system, a lean mixture sensor is provided for generating an analog current proportional to the lean air-fuel mixture in an exhaust pipe of an engine. Feedback control of the air-fuel ratio of the engine is possible using the analog output of the lean mixture sensor, thereby attaining an arbitrary air-fuel ratio on the lean side.
Additionally, a switch may be provided at a throttle valve within an intake air passage of the engine to detect the opening of the throttle valve. With this additional switch, the aimed air-fuel ratio is calculated in response to the on- and off-states of the switch. Feedback control of the air-fuel ratio of the engine is carried out so that the controlled air-fuel ratio is brought close to the aimed air-fuel ratio. When the switch is on, feedback control of the air-fuel ratio is stopped. Instead, the air-fuel ratio of the engine is controlled to be a power fuel increment air-fuel ratio, for example, about 12 to 13. A valve, such as a swirl control valve (SCV) may also be controlled in response to the on- and off-states of the switch.
During steady engine operation where the throttle valve is near the opening position detected by the switch, however, repeated on- and off-operation of the switch can result in fluctuation of the engine air-fuel ratio, thus causing surging. Also, an SCV is controlled by negative pressure having hysteresis characteristics. When an SCV is controlled by such a switch, it remains in an open state for a period of time even after the switch is changed from the on-state to the off-state. In this time period, the air-fuel ratio of the engine is erroneously controlled to be on the lean side, inviting misfiring or surging.